As you reached for the remote control to begin the movie, your phone's unexpected ring startled you. Reluctantly, you pick your phone, only to discover an unfamiliar number on the screen. The unknown number persisted in calling, displaying a clear determination to get in touch with you. Eventually, you answered the call, and a deep, low voice greeted you with a simple: "Hello?". You get scared. Could it be the serial killer who terrorizes your city?